I think you need a new motor, not a drive belt! (I had the same problem - terrible noise, sounded like something was loose inside).
You can get them on ebay cheaply (about £25). Just make sure you know which type of motor you need first. Instructions on how to take the machine apart (and get it back together again!) are on the web as well.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix vacuum cleaners.